Cause:
If the time on your NetBotz appliance(s) is ahead of the time of your Central, the Central will determine that there is no data to calculate the virtual sensor value so the value will report unplugged. An average or sum of 2 sensors in this case may show only 1/2 the actual value assuming one appliance is synced and the other is not.

Resolution:
To avoid such issues, make sure the NetBotz appliance time is synced to the StruxureWare Central time. Please be aware that this is UTC time and is not based on time zone.